The invention generally applies to the conversion of synthesis gas. Synthesis gas (or “syngas”) in the context of the invention refers to a mixture comprising carbon monoxide and hydrogen. Synthesis gas generally also comprises CO2. For use in the production of olefins by means of a Fischer-Tropsch process, the CO2 is preferably removed, reduced, or otherwise adjusted so as to provide the desired H2: CO ratios discussed below.
Synthesis gas is generally produced by methods such as steam reforming of natural gas or heavier hydrocarbons to produce hydrogen, or the gasification of coal, of biomass, and in some types of waste-to-energy gasification facilities. Particularly with reference to its potential biomass and waste origin, synthesis gas is increasingly receiving attention as an environmentally friendly, sustainable, resource of carbon-based chemicals.
Useful applications of synthesis gas will generally require chemical conversion of the gaseous CO and H2 components into hydrocarbons such as fuels or monomers, e.g. by Fischer-Tropsch synthesis.
The Fischer-Tropsch process is a catalyzed chemical reaction in which synthesis gas is converted into a range of hydrocarbons of various forms. The most common catalysts are based on iron and cobalt, although nickel and ruthenium have also been used. The principal purpose of this process is to produce a synthetic petroleum substitute, typically from coal, natural gas or biomass, for use as synthetic lubrication oil or as synthetic fuel.
The Fischer-Tropsch process involves a variety of competing chemical reactions, which lead to a series of desirable products and undesirable byproducts. When using cobalt catalysts, the most important reactions are those resulting in the formation of alkanes. These can be described by chemical equations of the form:(2n+1)H2+nCO→CnH(2n+2)+nH2Owith ‘n’ being a positive integer. Since methane (n=1) is mostly considered an unwanted by-product, process conditions and catalyst composition are usually chosen to favor higher molecular weight products (n>1) and thus minimize methane formation. In addition to alkane formation, competing reactions result in the formation of alkenes, as well as alcohols and other oxygenated hydrocarbons. Usually, only relatively small quantities of these non-alkane products are formed, although iron-based catalysts favoring some of these products have been developed. The formation of alkenes generally is within the following limiting chemical equations (one being to the extreme of water formation, the other to the extreme of carbon dioxide formation):2nH2+nCO→CnH2n+nH2OornH2+2nCO→CnH2n+nCO2 
Generally, the Fischer-Tropsch process is operated in the temperature range of 150-300° C. Higher temperatures lead to faster reactions and higher conversion rates, but also tend to favor methane production. As a result the temperature is usually maintained at the low to middle part of the range in the case of cobalt catalysts. Iron is usually employed at the higher end of the temperature range. Increasing the pressure favors the formation of long-chain alkanes, which is usually favorable for fuel production, but generally undesirable for the production of chemicals.
To the extent that Fischer-Tropsch processes have been described, this has very much focused on the production of fuels, i.e. selectivity towards an appropriate distribution of paraffins, e.g. with a view to providing desired fuel properties. This is quite a different field from the production of olefins, let alone lower olefins.
With the currently increasing attention for the use of sustainable resources of chemicals, and the use of biomass and waste streams, it is desired that synthesis gas can be put to use in a more versatile manner than the past focus on fuels. Thus it is desired to employ synthesis gas as a source of carbon also for chemicals.
Lower olefins are widely used in the chemical industry. They are mainly produced via naphtha and gas oil cracking, via paraffins dehydrogenation, or via FCC (fluid catalytic cracking). Environmental, economic and strategic considerations have encouraged the search of alternative feedstocks for the production of lower olefins. Different options have been considered such as natural gas, coal and biomass. In view hereof it is desired to provide a technically feasible and commercially attractive process to convert synthesis gas into lower olefins.
The invention preferably pertains to a specific Fischer-Tropsch process, viz. one that has been modified in order to yield lower olefins. Lower olefins, in the context of the invention, are straight-chain or branched alkenes having from 2 to 8, preferably from 2 to 6 carbon atoms, and most preferably this refers to C2-C4 alkenes. This process involves the use of supported iron-based catalysts, and reaction temperatures higher than 270° C., preferably higher than 300° C.
A supported catalyst is known to the person skilled in heterogeneous catalysis as a catalyst comprising a catalytically active part and a catalytically non-active part, wherein the catalytically non-active part (the support) generally forms the majority of the catalyst. This distinguishes a supported catalyst from a bulk-catalyst, in which the catalytically non-active part generally is the minority.
A reference to the selective hydrocondensation of carbon monoxide to light olefins, using alumina supported iron catalysts, is J. Barrault et al., React. Kinet. Catal. Left., Vol. 15, No. 2, 153-158 (1980). This document indicates that, by changing the support, the catalyst activity can be enhanced, and light olefin selectivity can be increased. However, the results attained by Barrault are representative of a moderate light olefin selectivity only, and by far insufficient suppression of methane production. In fact, Barrault sets out a particular problem, in that the most active catalysts are also the least selective.
Another reference is WO 84/00702. Herein iron nitrate, together with a praseodymium promoter, is used on a modified (heat treated) γ-alumina support. The catalyst is employed in the Fischer-Tropsch process, wherein it is asserted that C2-8 hydrocarbons are produced preferentially over methane. It is further indicated that a significant proportion of the hydrocarbons formed are 1-alkenes. The process is not, however, suited to selectively produce lower olefins over saturated hydrocarbons and, yet, keep methane production and higher olefins' production low. The heat-treated support has an α-alumina part and a γ-alumina part. The iron-containing particles are not detectably present on the α-alumina part.
Another reference that aims at producing lower olefins from synthesis gas, is DE 25 36 488 (1976). Herein an iron-based bulk catalyst is provided (iron with an oxide of titanium, zinc oxide and potassium oxide). Although, allegedly, this results in a process of high selectivity, with methane production only 10% and about 80% of lower olefins, the results therein are irreproducible and do not in fact lead to any suitable selectivity or reactivity.
Whilst all of the foregoing references represent unsuitable processes going back approximately 20-30 years in time, more recent development have not led to any success either, in terms of selective lower olefin production, effective suppression of methane production, and attractive catalyst activities or stabilities.
As to the latter, the chemical stability of the catalyst is an important issue in Fischer Tropsch processes. A chemically stable catalyst will be less prone to deactivation. In Fischer Tropsch processes, particularly with iron based catalysts and under conditions of temperature and pressure that favor alkene formation, catalyst deactivation is a serious problem. This is mainly due to coke formation, i.e. the undesired accumulation of carbon on the catalyst.
Thus, e.g., Sommen et al., Applied Catalysis 14 (277-288), 1985, describe Fischer-Tropsch catalysts that consist of iron oxide supported on activated carbon. Whilst these catalysts show an improved selectivity balance for lower olefins versus methane, the tested catalysts display a fast deactivation, i.e. they suffer from a low stability. Another drawback associated with activated carbon, is that it is prone to gasification, particularly at higher pressures and prolonged reaction times.
In WO 2009/013174 a promoted bulk iron catalyst is intended to be used in high temperature Fischer-Tropsch (340° C.) to produce lower olefins. Although methane selectivity of the claimed catalyst is low, the selectivity towards light olefins is insufficient. Moreover, the invention judiciously seeks to avoid bulk catalysts, and is specifically directed to supported catalysts.
Another issue with Fischer Tropsch catalyst performance is mechanical stability, e.g. vulnerability to fragmentation of catalyst particles related to extensive carbon laydown. This mechanical instability generally is an issue with bulk catalysts, particularly under conditions of increased catalyst activity, such as the elevated pressures typically used in industry. A reference in this regard is Shroff et al. Journal of Catalysis 156 (185-207), 1995. It is, inter alia, for this reason that the invention is specifically in the field of supported catalysts.
The present invention seeks to provide a Fischer-Tropsch route to lower olefins that has one or more of the following advantages:                a high selectivity for lower olefins, at cost of saturated hydrocarbons (paraffins), and higher olefins;        an effective suppression of the production of methane (i.e. a low selectivity for methane);        an increased catalytic activity, particularly without detracting from the results on selectivity.        a good chemical and mechanical stability, and particularly retaining this stability at elevated pressures as used in industry        a low amount of coke formation        